John P. commented at the Underground Bunker with more info about the decline in the last 5 years:

The TL;DR version: when you cut up the numbers, things are even worse.

After looking at the data in a bunch of different ways, I think the biggest predictor of whether someone is staff or whether they are public is going to be whether they are an Australian citizen or not. I think it is reasonable to assume that substantially all of the non-citizen Scientologists in Australia are staff. The incidence of non-citizen Scientologists as a percentage of total membership is dramatically higher than the incidence of non-citizens in the Australian population as a whole.

In 2011, there were 394 non-citizen Scientologists in Australia. In 2016, the number dropped to 298, a decline of 15%, less than the overall 21% decline that Tony reported yesterday. But there were 1,722 Scientologists who were citizens in 2011, which dropped to 1,349 in the 2016 census. That’s a decline of 22%. So, to my surprise, the number of public is collapsing. I had figured that a much greater percentage of staff (the non-citizens) would have left in the last five years, but that appears not to be the case.

As an aside, this suggests that staff in Australia is around 25% of total membership (non-citizens are 20% of total membership, but there are also undoubtedly a fair number of native staff members). This cross-checks nicely with estimates that I have been saying for some time that worldwide staff is approximately 5,000 out of total membership of 20,000 or so.

One other interesting data point: the age brackets that showed the greatest decline surprised me. Fortunately, the census data tracks in five-year buckets, so the bucket that was aged 15-19 in the 2011 census conveniently matches up to the 20-24 bucket in 2016, etc. It turns out that the the greatest decline in members who are Australian citizens between 2011 and 2016 is in the group that is now 25-29 years old, a population that dropped by 51% in the last five years. The second biggest drop was in the 30-34 bucket, down 29%, and those from 35-45 were also down about 28%. Older members, aged 50+, generally showed lower rates of declines.

It sounds like the church is losing second-generation members (or those who may have joined in college) far more than it is losing older first-generation believers, which surprises me, especially as older members would be dying at a higher rate than the younger ones, adding a few points to the rate of decline. The younger demographic is, of course, essential to any religion in part because they are starting families, and also because expansion there is necessary to replace older members that died. And that’s just not happening.

All in all, this suggests that the state of Scientology in Australia is pretty grim. Some other work that I had done a couple of weeks ago looking at the locations of people who signed the “stop Leah Remini” petition suggested that Scientology is also doing badly in the UK, another traditional stronghold. Unfortunately, the UK only holds census every decade, and the next one isn’t until 2021, so we won’t be able to prove the degree of implosion there for a few more years.